[PATCH 4/5] mm: hugetlb: replace hugetlb_free_vmemmap_enabled with a static_key

[Muchun Song]

The page_head_if_fake() is used throughout memory management and the
conditional check requires checking a global variable, although the
overhead of this check may be small, it increases when the memory
cache comes under pressure. Also, the global variable will not be
modified after system boot, so it is very appropriate to use static
key machanism.

Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
---
 include/linux/hugetlb.h    |  6 +++++-
 include/linux/page-flags.h |  6 ++++--
 mm/hugetlb_vmemmap.c       | 13 +++++++------
 3 files changed, 16 insertions(+), 9 deletions(-)

diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
index f7ca1a3870ea..876a1af73bec 100644
--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h
@@ -1057,7 +1057,11 @@ static inline void set_huge_swap_pte_at(struct mm_struct *mm, unsigned long addr
 #endif	/* CONFIG_HUGETLB_PAGE */
 
 #ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
-extern bool hugetlb_free_vmemmap_enabled;
+DECLARE_STATIC_KEY_MAYBE(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON,
+			 hugetlb_free_vmemmap_enabled_key);
+#define hugetlb_free_vmemmap_enabled					 \
+	static_branch_maybe(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON, \
+			    &hugetlb_free_vmemmap_enabled_key)
 #else
 #define hugetlb_free_vmemmap_enabled	false
 #endif
diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index d29c16f7b176..f05f806b98b4 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -183,7 +183,8 @@ enum pageflags {
 #ifndef __GENERATING_BOUNDS_H
 
 #ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
-extern bool hugetlb_free_vmemmap_enabled;
+DECLARE_STATIC_KEY_MAYBE(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON,
+			 hugetlb_free_vmemmap_enabled_key);
 
 /*
  * If the feature of freeing some vmemmap pages associated with each HugeTLB
@@ -222,7 +223,8 @@ extern bool hugetlb_free_vmemmap_enabled;
  */
 static __always_inline struct page *page_head_if_fake(const struct page *page)
 {
-	if (!hugetlb_free_vmemmap_enabled)
+	if (!static_branch_maybe(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON,
+				 &hugetlb_free_vmemmap_enabled_key))
 		return NULL;
 
 	/*
diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
index 527bcaa44a48..a382e12f6ff0 100644
--- a/mm/hugetlb_vmemmap.c
+++ b/mm/hugetlb_vmemmap.c
@@ -188,9 +188,9 @@
 #define RESERVE_VMEMMAP_NR		1U
 #define RESERVE_VMEMMAP_SIZE		(RESERVE_VMEMMAP_NR << PAGE_SHIFT)
 
-bool hugetlb_free_vmemmap_enabled __read_mostly =
-	IS_ENABLED(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON);
-EXPORT_SYMBOL(hugetlb_free_vmemmap_enabled);
+DEFINE_STATIC_KEY_MAYBE(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON,
+			hugetlb_free_vmemmap_enabled_key);
+EXPORT_SYMBOL(hugetlb_free_vmemmap_enabled_key);
 
 static int __init early_hugetlb_free_vmemmap_param(char *buf)
 {
@@ -204,9 +204,9 @@ static int __init early_hugetlb_free_vmemmap_param(char *buf)
 		return -EINVAL;
 
 	if (!strcmp(buf, "on"))
-		hugetlb_free_vmemmap_enabled = true;
+		static_branch_enable(&hugetlb_free_vmemmap_enabled_key);
 	else if (!strcmp(buf, "off"))
-		hugetlb_free_vmemmap_enabled = false;
+		static_branch_disable(&hugetlb_free_vmemmap_enabled_key);
 	else
 		return -EINVAL;
 
@@ -284,7 +284,8 @@ void __init hugetlb_vmemmap_init(struct hstate *h)
 	BUILD_BUG_ON(__NR_USED_SUBPAGE >=
 		     RESERVE_VMEMMAP_SIZE / sizeof(struct page));
 
-	if (!hugetlb_free_vmemmap_enabled)
+	if (!static_branch_maybe(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON,
+				 &hugetlb_free_vmemmap_enabled_key))
 		return;
 
 	vmemmap_pages = (nr_pages * sizeof(struct page)) >> PAGE_SHIFT;
-- 
2.11.0